Most of the power and usefulness of today's digital IC device s can be attributed to the increasing levels of integration. More and more components (resistors, diodes, transistors, and the like) are continually being integrated into the underlying chip, or IC. The starting material for typical ICs is very high purity silicon. The material is grown as a single crystal. It takes the shape of a solid cylinder. This crystal is then sawed (like a loaf of bread) to produce wafers typically 10 to 30 cm in diameter and 250 microns thick.
The geometry of the features of the IC components are commonly defined photographically through a process known as photolithography. Very fine surface geometries can be reproduced accurately by this technique. The photolithography process is used to define component regions and build up components one layer on top of another. Complex ICs can often have many different built up layers, each layer having components, each layer having differing interconnections, and each layer stacked on top of the previous layer. The resulting topography of these complex IC's often resemble familiar terrestrial "mountain ranges", with many "hills" and "valleys" as the IC components are built up on the underlying surface of the silicon wafer.
In the photolithography process, a mask image, or pattern, defining the various components, is focused onto a photosensitive layer using ultraviolet light. The image is focused onto the surface using the optical means of the photolithography tool, and is imprinted into the photosensitive layer. To build ever smaller features, increasingly fine images must be focused onto the surface of the photosensitive layer, i.e. optical resolution must increase. As optical resolution increases, the depth of focus of the mask image correspondingly narrows. This is due to the narrow range in depth of focus imposed by the high numerical aperture lenses in the photolithography tool. This narrowing depth of focus is often the limiting factor in the degree of resolution obtainable, and thus, the smallest components obtainable using the photolithography tool. The extreme topography of complex ICs, the "hills' and "valleys," exaggerate the effects of decreasing depth of focus. Thus, in order to properly focus the mask image defining sub-micron geometries onto the photosensitive layer, a precisely flat surface is desired. The precisely flat (i.e. fully planarized) surface will allow for extremely small depths of focus, and in turn, allow the definition and subsequent fabrication of extremely small components.
Chemical-mechanical polishing (CMP) is the preferred method of obtaining full planarization of a wafer. It involves removing a sacrificial layer of material using mechanical contact between the wafer and a moving polishing pad saturated with slurry. Polishing flattens out height differences, since high areas of topography (hills) are removed faster than areas of low topography (valleys). Polishing is the only technique with the capability of smoothing out topography over millimeter scale planarization distances leading to maximum angles of much less than one degree after polishing.
Prior art FIG. 1 shows a down view of a CMP machine 100 and prior art FIG. 2 shows a side view of the CMP machine 100. The CMP machine 100 is fed wafers to be polished. The CMP machine 100 picks up the wafers with an arm 101 and places them onto a rotating polishing pad 102. The polishing pad 102 is made of a resilient material and is textured to aid the polishing process. The polishing pad 102 rotates on a platen 104, or turn table located beneath the polishing pad 102, at a predetermined speed. A wafer 105 is held in place on the polishing pad 102 and the arm 101. The front side of the wafer 105 rests against the polishing pad 102. As the polishing pad 102 rotates, the arm 101 rotates the wafer 105 at a predetermined rate. The arm 101 forces the wafer 105 into the polishing pad 102 with a predetermined amount of down force. The CMP machine 100 also includes a slurry dispense arm 107 extending across the radius of the polishing pad 102. The slurry dispense arm 107 dispenses a flow of slurry onto the polishing pad 102.
The slurry is a mixture of deionized water and polishing agents designed to chemically aid the smooth and predictable planarization of the wafer. The rotating action of both the polishing pad 102 and the wafer 105, in conjunction with the polishing action of the slurry, combine to planarize, or polish, the wafer 105 at some nominal rate. This rate is referred to as the removal rate. A constant and predictable removal rate is important to the uniformity and throughput performance of the wafer fabrication process. The removal rate should be expedient, yet yield precisely planarized wafers, free from surface anomalies. If the removal rate is too slow, the number of planarized wafers produced in a given period of time decreases, hurting wafer throughput of the fabrication process. If the removal rate is too fast, the CMP planarization process may not be as precise as desired (leading to dishing, erosion, overpolishing, etc.), hurting the yield of the fabrication process.
Referring still to FIG. 1 and FIG. 2, as described above, the polishing action of the slurry and polishing pad 102 determines the removal rate and the removal rate uniformity, and thus, the effectiveness of the CMP process. Process engineers have discovered that in order to obtain sufficiently high and sufficiently stable removal rates, a large number of blanket wafers need to be processed on a respective CMP machine in order to properly calibrate the CMP process of the machine (e.g., "break-in" the machine's polishing pad, calibrate the slurry delivery rate, adjust wafer down-pressure, etc.). Each of these blanket wafers will typically show different removal rates as they are processed.
For example, in the case of a tungsten interconnect layer planarization process on CMP machine 100, the first of a batch of wafers show very low removal rates. The later processed wafers show much higher removal rates. Each successive wafer processed shows an incrementally higher removal rate. For a typical process, a large number of wafers will need to be processed in order for the removal rate of the tungsten layer of the wafers sufficiently increases, and perhaps more importantly, nominally stabilizes at a specified level. Until the removal rate of CMP machine 100 is sufficiently stabilized (e.g., calibrated to some nominal removal rate for each successive wafer) CMP machine 100 is unsuitable for device fabrication processing. Any fabricated wafer processed by CMP machine 100 and polishing pad 102 would have unpredictable planarity and film thickness, and hence would include many non-functional or unreliable integrated circuit dies, and accordingly, a relatively low yield.
Consequently, conventional CMP calibration methods include the processing of a large number of "test wafers" (or blanket wafers) using the target CMP machine (e.g., CMP machine 100). The tests are designed to obtain the various parameters which described the efficiency of the CMP process. To such parameters are the removal rate of the film material to be removed from the wafer and the uniformity in the material removal. These two parameters are a basic indicator of the quality of the CMP process. The removal rate will mainly be used to determine the polishing time of product wafers. The uniformity in material removal directly affects the global planarity across the wafer surface, which becomes more important as larger wafers are used in the fabrication of devices. Both the removal rate and the uniformity depend on consumables and polishing parameters including the pressure or down force, the speed of the polishing table, the speed of the wafer carrier, the slurry flow, and others. After a combination of consumables are chosen, the polishing parameters can be adjusted to achieve expected removal rate and uniformity.
There are significant drawbacks with this conventional calibration process. Because of several potential variations in production wafers (i.e., wafers with different devices) and in CMP process itself (e.g., the constant changing of the removal rate with pad wear), an end-point detector is widely used. Ideally, with in situ end-point detection, the stopping of a CMP process can be triggered at correct time regardless of the actual polishing rate. For example, if the polishing rate is lowered because of certain variations in the process, the end point will appear later and the polishing process will last longer. As a matter of fact the polishing time should also be related to the uniformity in material removal. In general, for example, with a metal CMP process (which removes excess metal film and ends the wafer surface with metal lines, or vias, surrounded by dielectric material) has a high non-uniformity, it will be required to have a long over-polish time in order to ensure the excess film is cleared across the wafer surface. On the other hand, if the CMP process has a very low non-uniformity, it will be affordable to have a little over-polish because the excess film clears at the same time across the wafer surface. Currently, the end-point method is mostly used to monitor the amount of film removal but not the non-uniformity. Additionally, the end-point detection only controls one variable, the polishing time, without providing any insight or control of the various other process variables. This greatly increases the number of test wafers needed in order to achieve a stable process.
Another drawback is the fact that there is a significant cost associated with these test wafers. In addition to the cost of the test wafers, there is a significant time penalty associated with breaking-in each new polishing pad and otherwise properly calibrating the CMP process of the machine. To attain a nominal removal rate (e.g., 4000 to 5000 Angstroms per minute) 20 to 50 test wafers must be processed, where each wafer consumes a valuable amount of processing time. In addition, the processing of test wafers subtracts from the useful life of the polishing pad 102 since it only has a finite amount of polishing cycles before it requires a change out. Another drawback of this conventional method of breaking in polishing pad 102 is the uncertainty associated with the number of test wafers which need to be processed in order to properly break-in a respective polishing pad.
Thus, what is required is a system which greatly reduces the number of test wafers required for properly calibrate a CMP process. What is required is a system which reduces the cost associated with achieving a stable CMP process. What is further required is a system which decreases the amount of process time and consumables required to qualify a fabrication line CMP machine. The present invention provides a novel solution to the above requirements.